Can you be more precise? When you say "the date is 30 or 60 or 90 days before today's date", do you mean exactly 30, or 60, or 90 days, or (say) 30 - 59, 60 - 89, or 90+ days? Do you want different colors for each date or date range? Later you say just yellow for expiring 30 days from today.

You can use a formula instead of a value comparison (as in above examples) to check for blank cells and ignore conditional formatting.

I've expanded on your question by creating an example workbook that applies three levels of conditional formatting:
Red: if Expiry Date is 90+ days old
Orange: if Expiry Date is 60+ days old
Yellow: if Expiry Date is less than or equal to 30 days from today's date

The rule are applied in the order stated above; if the first condition is true, the conditional formatting stops there, otherwise, it continues down the list.
The rules are:
Red: =IF(AND(A2<>"",A2<=TODAY()-90),TRUE,FALSE)
Orange: =IF(AND(A2<>"",A2<=TODAY()-60),TRUE,FALSE)
Yellow: =IF(AND(A2<>"",A2<=TODAY()+30),TRUE,FALSE)

Note that the formulas use relative referencing (A2, not $A$2) so that you can just expand the range to which is applies and still have the formatting work.

My example also has an aging column to the right as a check to ensure that the proper formatting applies.

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